WO 2011/080606 A1 discloses a brachytherapy apparatus comprising an applicator having at least one radiation source or seed receiving channel configured for being implanted in soft tissue adjacent a target region to be irradiated. The brachytherapy apparatus further comprises a means for generating a high resolution planning image of the target region including the applicator, wherein the high resolution planning image is used for determining a three-dimensional treatment plan. A tracking device tracks a position of the applicator relative to the target region, wherein the tracking device is configured to track the position of the applicator by measuring, via shape sensing, a location and a shape of the at least one radiation source or seed receiving channel.
During and after the at least one radiation source or seed receiving channel has been implanted in the soft tissue, the soft tissue swells, thereby modifying the position of the at least one radiation source or seed receiving channel relative to the target region. This changing of the relative position can reduce the quality of the brachytherapy.